1. Field of the Invention
The present invention relates to a stack-type piezoelectric device used as a drive source of an injector.
2. Description of the Related Art
The injector (fuel injector) of the internal combustion engine of an automotive vehicle is configured, for example, in such a manner that, by moving the valve body of a three-way valve or a two-way valve connected to a common rail having stored therein a high-pressure fuel, a fuel path is switched between open and closed states thereby to change the pressure applied to the nozzle needle and, by thus opening the nozzle needle, the fuel is injected.
A solenoid valve or the like is generally used as a drive source for activating the valve body. On the other hand, for the purpose of controlling the fuel injection accurately by controlling the drive source in finely detailed fashion, and as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-229993, for example, an attempt has been made to use a stack-type piezoelectric device as a drive source for the injector.
Nevertheless, an injector using the piezoelectric device as a drive source, though proposed as described above, has yet to be practically implemented.
Depending on the type of the injector, fuel must be injected under a pressure as high as more than 100 MPa. Thus, the piezoelectric device for the injector is required to have high reliability in a harsh operating environment.
Further, the piezoelectric device for the injector which opens/closes the valve at high speed requires a very quick response. The piezoelectric device for the injector charges and discharge in a short time, and a large current flows therein. With the conventionally known piezoelectric device, therefore, the energy consumption is so large that a large burden is imposed on the control circuit and the circuit configuration becomes undesirably bulky.
A typical configuration of the piezoelectric device for the injector is described below.
Specifically, the piezoelectric device for the injector comprises a plurality of piezoelectric layers which expand or contract in accordance with the applied voltage and a plurality of internal electrode layers for supplying the applied voltage, the piezoelectric layers and the inner electrode layers being stacked alternately with each other, and a pair of side electrode layers formed on the outer peripheral sides, respectively, of the piezoelectric device for the injector which are alternately electrically energized in such a manner that adjacent ones, with a piezoelectric layer therebetween, of the internal electrode layers alternately assume different polarities, wherein a pair of the outer electrode layers each include a terminal for supplying the applied voltage from an external power supply.
In a harsh operating environment, the outer electrode layers are liable to a crack or become disconnected, and it becomes impossible to supply the applied voltage to the internal electrode layers, with the result that the performance and the function, of the piezoelectric device for the injector, are liable to be adversely affected.
Japanese Unexamined Patent No. 10-229227 proposes, in addition to a single outer electrode layer, a three-dimensional structure of electrodes formed through partial contact points of the single outer electrode layer.
With this configuration, even in the case that the outer electrode layer develops a crack or is disconnected, the three-dimensionally structured electrodes can secure the electric conduction of the outer electrode layer.
In view of the very harsh operating environment of the injector, however, the partial contact points are liable to become disconnected and also have an insufficient practical durability.
Further, in the case where a plurality of cracks develop in the outer electrode layer between adjacent partial contact points, current fails to flow between the cracks, resulting in an electrical disconnection.
On the other hand, Japanese unexamined Patent Publication No. 59-204288 proposes a piezoelectric device of a different configuration.
In this piezoelectric device, a material like a conductive rubber is continuously coated on the side thereof, and the piezoelectric device has a configuration including a plurality of piezoelectric short plates each 0.5 mm thick stacked one on another. The inner layer electrode extends to the side surfaces of the short plates.
With the piezoelectric device described above, however, the piezoelectric layer has a thickness of only about 100 xcexcm and therefore it is difficult to extend the inner layer electrode to the side surfaces thereof. Thus, it is difficult to positively secure the electrical connection between the first outer electrode and the inner layer electrode with a conductive rubber-like material and an ordinary method of baking metal is required.
The present invention has been achieved in view of the problems in the prior art described above, and an object thereof is to provide a piezoelectric device most suitably used as a drive source for the injector in a harsh environment or, in particular, to a piezoelectric device for the injector which consumes less energy and has a faster response.
According to a first aspect of the invention, there is provided a piezoelectric device, built in an injector for generating the driving force for the injector, comprising:
a plurality of piezoelectric layers adapted to expand or contract in accordance with an applied voltage and a plurality of inner electrode layers for supplying the applied voltage, the piezoelectric layers and the inner electrode layers being stacked alternately with each other;
a pair of first outer electrode layers formed on the outer peripheral sides, respectively, of the piezoelectric device for the injector, which outer electrode layers are adapted to be alternately electrically energized so that adjacent ones of the inner electrode layers, with a piezoelectric layer therebetween, assume different polarities; and
second outer electrode layers are formed on the first outer electrode layers, respectively, and are made of a conductive material having a larger breaking elongation than the first outer electrode layers.
One aspect of the invention is that a second outer electrode layer made of a conductive material having a larger breaking elongation than a first outer electrode layer is formed on each of a pair of the first outer electrode layers.
The operation of this aspect of the invention will be explained below.
Each of the second side electrode layers has so large a breaking elongation that it is not easily cracked or disconnected even in a harsh operating environment. The cracking or disconnection of the electrode layer is caused by the expansion of the piezoelectric device which generates the stress in the piezoelectric device for the injector or the electrode layers.
In the configuration according to this aspect of the invention, even in the case where the first outer electrode layers are cracked and disconnected, the electrical conduction between the first outer electrode layers and the inner electrode layers is secured through the second outer electrode layers which have a large breaking elongation and are not easily cracked or disconnected. As a result, a piezoelectric device for the injector can be produced which is usable even in a harsh operating environment.
Also, the reliability can be improved by providing substantially two outer electrode layers including a first outer electrode layer and a second outer electrode layer. With the first outer electrode alone, a slight cracking or disconnection would grossly deteriorate the reliability of the piezoelectric device for the injector. With the configuration according to this aspect of the invention, however, the provision of two outer electrode layers including the second outer electrode layer permits either the first or second outer electrode layer to apply a voltage to the inner electrode layers.
The breaking elongation is determined by the value based on the dumbbell elongation measuring method for a rubber material specified in JIS K6301.
As described above, according to this aspect of the invention, a piezoelectric device most suitably applicable as a drive source of an injector can be provided.
According to a second aspect of the invention, there is provided a piezoelectric device, for an injector in which the second outer electrode layers are preferably configured of a conductive material having a breaking elongation of 1.5% or higher.
This produces a piezoelectric device for the injector having a superior durability capable of enduring protracted operation.
A breaking elongation of less than 1.5% would generate a cracking due to the fatigue derived from the drive operation.
According to a third aspect of the invention, there is provided a piezoelectric device, for an injector, in which the second outer electrode layers are preferably configured of a mixture of a metal material and a resin material.
As a result, second outer electrode layers having a large breaking elongation can be produced.
The stress imposed on the second outer electrode layers by the expansion/contraction of the piezoelectric device for the injector can thus be relaxed and a cracking rarely occurs.
According to a fourth aspect of the invention, there is provided a piezoelectric device, for an injector, in which the resin material of the mixture is preferably at least selected one of silicone, epoxy, polyimide and vinyl phenol.
The mixture of any of these substances can produce the second outer electrode layers having a superior heat resistance and a large breaking elongation. The temperature of the operating environment of the injector used with the automotive internal combustion engine is as high as not lower than 150xc2x0 C. The aforementioned piezoelectric device having the second outer electrode layers containing the resin material can reduce the stress on the second outer electrode layers generated by the expansion of the piezoelectric device, and the layers rarely develop cracks due to a superior heat resistance.
According to a fifth aspect of the invention, there is provided a piezoelectric device, for an injector, in which the first or second outer electrode layers preferably are composed of a metal material containing Ag.
Silver has a high melting point and a high electric conductivity and therefore rarely poses a problem of conduction failure. Thus, the conductivity between the first and second outer electrode layers can be secured in a high temperature environment. Especially in the case where the first outer electrode layers contains Ag, the conductivity can be secured between the first outer electrode layers and the inner electrode layers in a high temperature environment.
According to a sixth aspect of the invention, there is provided a piezoelectric device, built in an injector and for generating a drive force for the injector, comprising:
a plurality of piezoelectric layers adapted to expand or contract according to the applied voltage and a plurality of inner electrode layers for supplying the applied voltage, the piezoelectric layers and the inner electrode layers being stacked alternately; and
a pair of first side electrode layers formed on the outer peripheral side surface of the piezoelectric device for the injector and adapted to be alternately electrically energized so that adjacent ones of the inner electrode layers, with a piezoelectric layer therebetween, may have different polarities;
wherein the first outer electrode layers are coupled to metal members, respectively, through the electrically conductive second outer electrode layers having a larger breaking elongation than the first outer electrode layers.
The functions of this aspect of the invention will be explained below.
Since the second outer electrode layers have a large breaking elongation, cracks or disconnections are not easily generated even in a harsh operating environment.
With the configuration according to this aspect of the invention, even in the case where the first outer electrode layers develop cracks or are disconnected, the metal members arranged through the second outer electrode layers are not easily cracked or disconnected and can secure the electric conductance between the first outer electrode layers and the inner electrode layers. As a result, a piezoelectric device for the injection usable even in a harsh operating environment can be produced.
As described above, according to this invention, a piezoelectric device most suitable as a drive source for the injector can be provided.
The metal members and the first outer electrode layers may be bonded wholly to each other by the second outer electrode layers. Also, the space between the metal members and the first outer electrode layers may be filled with the second outer electrode layers.
According to a seventh aspect of the invention, there is provided a piezoelectric device, for an injector, in which the second outer electrode layers are preferably formed of a conductive material having a breaking elongation of not less than 1.5%.
As a result, a piezoelectric device, for an injector, which can stand protracted operation and has a high durability can be produced. If the breaking elongation is less than 1.5%, cracks may be generated by fatigue due to the drive operation.
According to an eighth aspect of the invention, there is provided a piezoelectric device for the injector, in which the second outer electrode layers are preferably configured of a mixture containing a metal material and a resin material.
This can produce a coupling member having a large breaking elongation.
As a result, the stress imposed on the coupling member by the expansion of the piezoelectric device for the injector is reduced, thereby making it difficult for cracks to develop.
According to a ninth aspect of the invention, there is provided a piezoelectric device, for an injector, in which the resin material of the mixture is preferably at least a selected one of silicone, epoxy, polyimide and vinyl phenol.
The use of any of these substances can produce second outer electrode layers having a superior heat resistance and a large breaking elongation. The temperature of the operating environment of the injector used with the automotive internal combustion engine is as high as not lower than 150xc2x0 C. The aforementioned piezoelectric device having the second outer electrode layers containing the resin material can reduce the stress on the second outer electrode layers generated by the expansion of the piezoelectric device, and rarely develops cracks due to a superior heat resistance. The other detailed points are similar to those of the fourth aspect of the invention.
According to a tenth aspect of the invention, there is provided a piezoelectric device, for an injector, in which the first or second outer electrode layers preferably are composed of a metal material containing Ag.
Silver has a high melting point and a high electric conductivity and therefore rarely poses a problem of conduction failure. Thus, the conductivity between the first and second outer electrode layers can be secured in a high temperature environment. Especially in the case where the first outer electrode layers contain Ag, the conductivity can be secured between the first outer electrode layers and the inner electrode layers in a high temperature environment.
According to an eleventh aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the metal members are each preferably formed of a metal plate having a corrugated section.
According to a twelfth aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the metal members are each preferably formed of an elastic member such as a spring member.
According to a thirteenth aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the metal members are each preferably formed of a metal plate having slits or holes.
This configuration can provide the metal members with an elasticity. With the piezoelectric device for the injector of which the length is changed along the stack height in accordance with the expansion/contraction of the piezoelectric layers, the expansion/contraction of the metal members simultaneous with the piezoelectric device can prevent the cracking of the metal members.
Also, metal members having a plurality of the aforementioned configurations combined may be used. For example, metal members having slits and having a corrugated section can be used with equal effect.
According to a 14th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injectors in which the metal members are each preferably formed of stainless steel or copper.
The use of a material having a large elasticity makes it difficult for fatigue failure to occur even when the metal members are extended by the elongation of the piezoelectric device for an injector.
A stainless steel containing, as main components, 18% Cr and 8% Ni can be used. Also, a copper material such as phosphor bronze containing, as main components, 90% Cu, 9% Sn and 0.35% P can be used.
According to a 15th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the metal members are each preferably embedded in the corresponding second outer electrode layer.
This configuration makes it possible to couple the metal members positively with the first outer electrode layers.
According to a 16th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which each metal plate having a corrugated section preferably has ridge portions protruded outward of the outer peripheral side of the piezoelectric device for the injector and valley portions each located between given each pair of ridge portions, and at least a part of the ridge portions and the valley portions are formed diagonally over the piezoelectric layers and the inner electrode layers constituting the piezoelectric device for the injector.
This configuration permits the metal plates to couple adjacent ones of the piezoelectric layers along the stack height. Thus, the conductivity of each inner electrode layer can be positively secured by the metal members even in the case where the first outer electrode layers develop cracks due to the expansion or contraction of the piezoelectric device for the injector.
According to a 17th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which each slit is preferably formed diagonally over the piezoelectric layers and the inner electrode layers constituting the piezoelectric device for the injector.
The provision of diagonal slits makes it possible for the metal plates to couple adjacent ones of the piezoelectric layers along the stack height. Thus, the conductivity of each inner electrode layer can be positively secured by the metal members even in the case where the first outer electrode layers develop cracks due to the expansion or contraction of the piezoelectric device for the injector.
According to an 18th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the distortion due to expansion and contraction is preferably not less than 0.05% and the number of drive cycles of the piezoelectric device for the injector is preferably at least 109. The piezoelectric device satisfying this requirement for distortion and the number of drive cycles can provide the performance and the durability required of an injector.
Specifically, the distortion of a piezoelectric device used as a drive source of the injector, due to expansion/contraction thereof, is preferably not less than 0.05%, if the valve body is to be driven.
For application to an internal combustion engine of an automotive vehicle, the piezoelectric device preferably has a number of drive cycles of not less than 109. The number of drive cycles is an index representing the maximum number of expansion/contraction cycles the piezoelectric device can be driven before an abnormality such as shorting occurs. The greater the number of drive cycles, the higher the durability of the piezoelectric device involved. For both the distortion and the number of drive cycles, the larger, the better.
According to a 19th aspect of the invention, there is provided a stack-type piezoelectric device built in an injector,
wherein side electrodes are arranged on the two sides of the piezoelectric device, respectively, and an external electrode is arranged on each of the side electrodes for establishing electric conduction with external parts,
wherein the external electrodes each includes a core member and a metal cover for covering at least a part of the core member, the external electrode being coupled to a part of the corresponding side electrode, and
wherein the specific electric resistance of the core member is not less than 5xc3x9710xe2x88x926 (xcexa9xc2x7cm), and the specific electric resistance of the metal cover is not more than one half of that of the core member.
Now, the functions and effects of this aspect of the invention will be explained.
According to this aspect of the invention, the external electrode includes the core member and the metal cover, and the specific electric resistance of these parts have the specific values, respectively, described above.
In the case where the specific electric resistance of the core member is less than 5xc3x9710xe2x88x926 (xcexa9xc2x7cm), a sufficient electric conductivity can be secured without providing the metal cover. In the case where the specific electric resistance of the core member is not less than 5xc3x9710xe2x88x926 (xcexa9xc2x7cm) as in this aspect of the invention, on the other hand, a sufficient electric conductivity cannot be secured. In such a case, the energy consumption during the conduction of the piezoelectric device is greatly increased.
In view of this, according to this aspect of the invention, each external electrode is so configured that at least a part of the core member is covered by the metal cover as described above, and the specific electric conductance of the metal cover is set to not more than one half of that of the core member. In the case where a core member having a specific electric resistance of 5xc3x9710xe2x88x926 (xcexa9xc2x7cm) is selected, for example, the specific electric resistance of the metal cover is set to not more than 5xc3x9710xe2x88x926 (xcexa9xc2x7cm)
As a result, the metal cover supplements the electrical conductivity of the core member, and therefore the electrical conductivity of the external electrode as a whole is remarkably improved. Thus, less energy is consumed by the electrical resistance of the external electrode, thereby making it possible to suppress the energy consumption of the whole piezoelectric device. In addition, the burden on the control circuit for controlling the piezoelectric device is reduced to allow a smaller circuit configuration.
In this way, according to this aspect of the invention, a piezoelectric device for the injector is provided which consumes less energy and is capable of a fast response.
According to a 20th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the core member is preferably a metal material having a tensile strength of not less than 500 MPa. Specifically, the core member is preferably made of a metal material having some degree of elasticity, i.e. a somewhat high elastic limit sufficient to follow the expansion or contraction of the piezoelectric device. In the case where the tensile strength is less than 500 MPa, the durability is liable to decrease in following the expansion or contraction of the piezoelectric device.
According to a 21st aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the core member is preferably made of selected one of SUS, copper beryllium, phosphor bronze and nickel silver. In this case, the core member can have a sufficient elasticity, thereby making it possible to improve the durability of the external electrode as a whole.
According to a 22nd aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which the metal cover is preferably made of selected one of silver, gold and copper. In this case, the specific electric resistance of the metal cover can be greatly reduced, thereby making it possible to further improve the electrical conductivity of the external electrode as a whole.
According to a 23rd aspect of the invention, there is provided a piezoelectric device built in the injector for generating a drive force for the injector, in which the metal cover is preferably arranged in such a manner as to cover at least 30% of the surface area of the core member. In the case where the area covered by the metal cover is less than 30% of the surface area of the core member, a problem is posed in that the electric resistance of the whole external electrode cannot be reduced sufficiently.
According to a 24th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which a substrate material for reducing the distance between the core member and the metal cover is preferably interposed between the core member and the metal cover. By doing so, the core member and the metal cover can be attached to each other more closely. The substrate material may be Ni plating, for example.
According to a 25th aspect of the invention, there is provided a piezoelectric device, built in an injector for generating a drive force for the injector, in which the outer electrodes are made of a conductive adhesive containing silver, and the metal cover is arranged on at least the joint surface between the external electrode and the conductive adhesive.
According to a 26th aspect of the invention, there is provided a piezoelectric device, built in an injector, for generating a drive force for the injector, in which each of the outer electrodes and the corresponding external electrodes are coupled to each other by a conductive adhesive containing silver, and the metal cover is arranged on at least the joint surfaces between the external electrodes and the conductive adhesive.
In any of these cases, an increase in electrical resistance (interface resistance) due to age degradation can be suppressed in the boundary surface between the external electrode and the conductive adhesive.